It has been claimed that O2 under pressures of 2-1/2 to 3 atmos. given to senile patients improve their cognitive abilities. The purpose of this study is to replicate this work in a controlled fashion. A total of 80 patients with an age range between 55-70 years will be used. The first group fo 40 patients will be treated in the hyperbaric chamber and another 40 patients would be treated with isobaric oxygen or air. The hyperbaric group of 40 patients will have a ratio of 1 experimental to 1 control patient. The treatment procedure will consist of breathing hyperbaric O2 twice daily for 15 days. Each session will last 90 minutes with a period of "surfacing" between the two dives. The control group will breathe a mixture of O2 and N2. The procedure for these therapies would be identical to those done routinely in the NYU chamber. The second group of 40 patients would be divided into 2 groups. One group will breathe isobaric oxygen and the other air. This procedure would be carried out in ward PQ3 at Bellevue.